OK, you replaced the RadTextBox control with RadComboBox, but what are the steps to reproduce the problem? I see that the combo doesn't have the AutoPostBack property set to true, so it will not fire a postback. Now if I remove the panel and the hidden button which are part of the workaround, there is no difference in the page's behavior no matter if there is RadFormDecorator on the page or not.
Can you please open a ticket and provide full, working solution along with steps to reproduce? This will prevent further misunderstandings and we will do our best to help you right away.
All the best,
the Telerik team